function add() {
    temp = parseInt(document.getElementById("num").innerHTML);

    temp = temp +1;

    document.getElementById("num").innerHTML = String(temp);
}

function jian() {
    if (parseInt(document.getElementById("num").innerHTML) > 0) {
        temp = parseInt(document.getElementById("num").innerHTML);

        temp = temp -1;

        document.getElementById("num").innerHTML = String(temp);
    }
    else{
        return 0;
    }
}